AIDS and Kaposi's Sarcoma
Treatment of cancers is an area of clinical medicine that is of vital importance, but yet remains fraught with complications and suboptimal results. There are various cancer treatments such as surgery, chemotherapy, and radiation therapy. Most of these treatments have serious side effects because of lack of specificity of the treatments.
Acquired Immunodeficiency Syndrome (AIDS) has become a serious and deadly epidemic that has claimed the lives of thousands of people. Currently, there are no completely effective vaccines to cure AIDS. The most common malignancy found in patients with AIDS is Kaposi's sarcoma (KS). The AIDS related form of Kaposi's sarcoma most frequently presents with cutaneous lesions. Mucosal involvement of the oral cavity is the second most common site of the disease. The tumor lesions are noted frequently on the palate and gums, and can cause tooth loss. See Paredes, J., J. Acquir. Immune. Defic. Syndr. Hum. Retriviral, 9(2): 138–144, 1995, which is herein incorporated by reference. Nearly every organ can be affected with Kaposi's sarcoma, including liver, lung, spleen, pancreas, and heart, etc.
Kaposi's sarcoma is a multifocal neoplasm, consisting of several cell types and angiogenesis. The tumor is usually considered to be of endothelial origin (Rutgers et al., Am. J. Pathol. 122: 493–499, 1986). Even though there have been attempts in curing Kaposi's sarcoma, current Kaposi's sarcoma treatments can cause serious side effects such as myelotoxicity and neurotoxicity. See Northfeldt, et al., Hematology/Oncology Clinics of North America 5: 297–310, 1991, which is incorporated herein by reference. Existing Kaposi's sarcoma treatments can induce immunosuppression, and thus can worsening the pre-existing immunodeficiency that is usually present in AIDS patients. In 1995, it was discovered that the growth of Kaposi's sarcoma derived cell lines was inhibited in vitro and in vivo by a human pregnancy hormone purified from urine, the human chorionic gonadotropin (hCG). Lunardi-Iskandar, Y., et al., Nature 375: 64–68, 1995. Later, researchers found that the active ingredient was not hCG, but instead it may have been contaminating hCG-associated factors that exhibit anti-KS activities. However, the precise nature of these factors is not known in the art at the present time, and there exists a great need to find the compound that inhibits the growth of KS cancer cells.
The RNase A Superfamily: Antitumor Agents, Sequence Homology.
The Ribonuclease A superfamily is consisted of ribonucleases that are endonucleases, and are homologous to each other in sequence. They have long been known to be cytotoxic agents during host defense and physiological cell death pathway. Roth, J. Cancer Res. 23: 657–666, 1963. As early as 1955, it was discovered that bovine pancreatic RNase A had anticancer properties when injected into mice tumor models. See Ledoux, L., Nature, 176: 36, 1955. Later, bovine seminal RNase, a member of the RNase A superfamily, and closely homologous to bovine pancreatic RNase A, was found to be toxic to mammalian cells in a number of systems. See Vescia, S. and Tramontano, D., Mol. Cell. Biochem., 36: 125, 1981. The bovine seminal RNase was also found to be active in extending the survival rate in tumor models. See Laccetti, O., et al., Cancer Res., 52: 4582, 1992.
Among the RNase A superfamily members, eosinophil derived neurotoxin protein (EDN) has been identified as a neurotoxin, along with other human RNases, such as eosinophil cationic protein (ECP). Both EDN and ECP are homologous to RNase A in about 35% of their amino acid sequences. Cytotoxic eosinophil proteins have been reported to have RNase activity (Slifman, et al., J. Immunol. Comm., 137(9): 2913–2917, 1986; Gullberg, et al., Biophys. Biochem. Res. Comm., 139(3): 1239–1242, 1986). The sequence of human EDN was found to be identical to that of the nonsecretory ribonuclease from human urine (Beintema, et al., Biochemistry, 27: 4530–4538, 1988). It was recently discovered that EDN polypeptides are also potent inhibitors of cell-free protein synthesis (Zewe, et al., Immuno Technology 3:127–136).
The N-terminus of the RNase A superfamily polypeptides is important for expression of their cytotoxic properties. See, Boix, E., et al., J. Mol. Biol., 257: 992–1007, 1996; Newton, D. L., et al., Protein Eng. 10: 463–470, 1997; Newton, D. L., et al., Biochemistry 37: 5173–5183, 1998. It was recently discovered that a protein isolated from the urine of pregnant women which is homologous to the N-terminal sequence of EDN shows inhibitory effect on the development of mouse oocytes in culture. Sakakibara, R., et al., Chem. Pharm. Bull. 39: 146–149, 1991.
The present invention is based upon the discovery that RNase A superfamily polypeptides which are modified at the amino terminal by adding a short amino acid sequence are effective in directing the cytotoxic activity of the RNase to cancerous endothelial cells. This discovery provides promising treatments for cancers, such as Kaposi's sarcoma.